A nitride semiconductor device is under active development for use as a semiconductor device with high-voltage resistance and high output, by making use of the feature of a high saturation electron speed, a wide band gap and the like. As for the nitride semiconductor device, there have been a large number of reports on a field effect transistor, and particularly on a high electron mobility transistor (HEMT). In particular, an AlGaN/GaN-HEMT has received attention, which uses GaN as an electron transit layer and AlGaN as an electron supply layer. In the AlGaN/GaN-HEMT, distortion occurs in AlGaN, which originates in the difference between the lattice constants of GaN and AlGaN. A high concentration of two-dimensional electron gas (2D EG) is obtained due to the piezo polarization generated by the distortion and the spontaneous polarization of AlGaN. Because of this, high-voltage resistance and a high output can be realized.